1. Field of the Invention
The present invention relates generally to satellite communications and, more particularly, interference cancellation in narrowband satellite systems.
2. Description of the Related Art
Efficient use of available bandwidth in wireless, such as satellite, communications applications is a problem of paramount importance. This is particularly true of narrow band satellite applications wherein bandwidth is very limited. An example of such a narrow band satellite includes very small aperture terminal (VSAT) systems. VSAT systems use compact earth stations that are installed at one or more customer's premises to provide links among the premises over a wide coverage area. Typically, in such systems, remote ground terminals are used for communicating via a geosynchronous satellite from a remote location to a central hub station or other remote locations. The central hub station communicates with multiple remote ground terminals. VSAT systems are used to handle customer network requirements, from small retail sites up to major regional offices, and can support two-way data, voice, multi-media, and other types of data. A particular advantage of these systems is their relatively low site cost and small earth-station size.
In wireless systems, multiple users share the same bandwidth. Channel sharing through fixed-allocation, demand assigned or random-allocation modes is known as multiple access. Two of the more commonly known basic multiple-access techniques include time division multiple access (TDMA) and code division multiple access (CDMA).
VSAT type systems have traditionally implemented TDMA using time division multiplexed (TDM) mode. Such systems generally are used for low speed (300 bps to 19,200 bps) data communications such as credit card processing and verification, point-of-sale inventory control and general business data connectivity. A typical TDM/TDMA network, when implemented in a star topology (FIG. 1), uses a large satellite hub system that manages all network terminal access and routing. Data is transmitted to and from the hub 102 in short bursts on satellite channels that are shared with a number of other VSAT terminals 104A-104H. The hub 102 communicates with these VSAT terminals 104A-104H over a higher speed outbound TDM satellite carrier. The terminals 104A-104H transmit back to the hub 102 on assigned inbound carriers using TDM protocols. Such a combination enables a predetermined number of slots in time each second that each terminal 104A-104H can transmit data. In addition, more or less time can dynamically be assigned to the terminals 104A-104H based upon each terminal's individual requirements.
In contrast, in a CDMA type system a user's station signal is multiplied by a unique spreading code at a high speed to be spread in a wide frequency band. Thereafter, the signal is transmitted to a transmission path. In a receiving side, the signal that was multiplexed by the spreading code is subjected to a despreading process to detect a desired signal. Signal detection is based on a unique spreading code assigned to a user's station. If despreading is carried out with reference to a particular code used to spread a transmission signal, a user's station signal is correctly reproduced.
Regardless of the access technique used, increased efficiency and lower cost is a primary goal. Accordingly, efficiencies in bandwidth may be realized using techniques such as crowding of adjacent channels, frequency re-use, and increasing of data rates, generally resulting in an increased amount of data traveling through the limited amount of available bandwidth. Unfortunately, however, such techniques introduce a significant amount of interference which must be cancelled. Known interference cancellation approaches exist for CDMA applications and may broadly be categorized into successive and parallel.
The successive approach generally requires highly accurate estimation of the received signal amplitudes as well as sorting the signals in decreasing order according to the powers. An alternative approach, parallel cancellation, subtracts off all the users' signals from all of the others and then the required data is detected. The parallel method outperforms the successive scheme when the users are received with equal strength, but at the expense of higher hardware complexity.
It is to be noted that, while similarities exist between a highly bandwidth-efficient satellite system and the problem of multi-user detection of spread-spectrum CDMA signals, all heretofore known detection methods utilize code sequences that are assigned to the different CDMA users and are therefore known beforehand. Unfortunately, such data is not available in a narrowband system, such as TDMA, making the problem of interference cancellation inherently more difficult.
Accordingly, there is a need for a system in narrow band type satellite applications that provides for efficient utilization of available bandwidth using a compensation scheme that eliminates interference and increases spectral efficiency.